Types of Sampling Plans

Functions

Types of Sampling Plans

In single sampling, a random sample of n items is selected from a lot of size N. If the number d of nonconforming (defective) items found in the sample is less than or equal to an acceptance number c, the lot is accepted. Otherwise, the lot is rejected.

In double sampling, a sample of size n₁ is drawn from the lot, and the number d₁ of nonconforming items is counted. If d₁ is less than or equal to an acceptance number a₁, the lot is accepted, and if d₁ is greater than or equal to a rejection number r₁, the lot is rejected. Otherwise, if a₁<d₁<r₁, a second sample of size n₂ is taken, and the number of nonconforming items d₂ is counted. Then if d₁+d₂ is less than or equal to an acceptance number a₂, the lot is accepted, and if d₁+d₂ is greater than or equal to a rejection number r₂=a₂+1, the lot is rejected. This notation follows that of Schilling (1982). Note that some authors, including Montgomery (1996), define the first rejection number using a strict inequality.

In Type A sampling, the sample is intended to represent a single, finite-sized lot, and the characteristics of the sampling plan depend on D, the number of nonconforming items in the lot, as well as N, n, and c.

In Type B sampling, the sample is intended to represent a series of lots (or the lot size is effectively infinite), and the characteristics of the sampling plan depend on p, the proportion of nonconforming items produced by the process, as well as n and c.

A hypergeometric model is appropriate for Type A sampling, and a binomial model is appropriate for Type B sampling.

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